The inlet of the inlet pipe for fueling an automobile is typically sealed by a fuel cap that is attached and detached by a rotation operation. This kind of fuel cap not only has poor operability because a plurality of rotation operations is required when fueling, but there was also the problem that when removed, it was necessary to come up with a means for remembering the place one left it or for preventing loss. To address this kind of problem, arts for which a flap valve mechanism is provided on the fuel cap are known such as in JP-A 2001-47876. The flap valve mechanism closes the inlet of the inlet pipe using the flap valve and also seals it by biasing using a spring, and during fueling, the inlet opens by pressing the flap valve in resistance to the biasing force of the spring using the fueling gun. Also, a pressure adjustment valve for regulating the internal pressure of the fuel tank is attached to the flap valve mechanism. However, with the flap valve mechanism, when the pressure adjustment valve is attached to the flap valve, not only does the mechanism become complex, but there was also the problem that the number of parts becomes high.